Materials that respond dynamically to environmental stimuli can be called intelligent or smart materials. Shape memory polymers are intelligent materials and there is increased interest in several industries. As a result, research on such materials is actively growing both in academic and industrial sectors. Although the term ‘smart material’ has been conventionally used, all materials are in general responsive (and in this sense smart) but whether they are responsive in an adaptive way is questionable. A ‘very smart’ adaptive response is exhibited if materials/material systems are able to respond dynamically to a number of input stimuli and if this response is repeatable. For example, a simple pressure transducer that produces a voltage dependent signal upon the input pressure in a direct one-to-one fashion could be regarded as ‘smart’ in a basic or simple way. However, a pressure transmitter incorporating a thermocouple that measures both temperature and pressure and corrects the pressure in response to the sensor's temperature coefficient could be regarded as ‘very smart’. (Lendlein, A. and Kelch, S. 2002)
SMPs have the capability of memorizing a permanent shape and one programmable for one or many temporary shapes, while spontaneously recovering their original permanent shapes from temporary deformations upon exposure to an external stimulus. As shown in FIG. 1, a closed flower (temporary shape) made of a SMP is fixed at a lower temperature and recovers from the closed state to an open flower (original shape) when the temperature is increased above its switch temperature. FIG. 1 Appearance recovery in magic mirrors (the appearance of an old woman recovers to her young appearance in a magic mirror) and shape recovery of SMPs (flower made of SMPs opens its petals at high temperature). (Hu, J. and Chen, S. 2010).
SMPs have been around since the mid-1980s. They can be stimulated by temperature, pH, chemicals, and light, and are defined as polymer materials with the ability to sense and respond to external stimuli in a predetermined way. Polymer materials have various elasticities from hard glass to soft rubber. SMPs, however, have the characteristics of both hard and soft materials, and their elasticity modulus shows reversible change with the transition temperature. Upon application of an external stimulus, they also have the ability to change their shape. Among these SMPs, the shape of thermally responsive shape memory polymers can be readily changed above the shape memory transition temperature (Ttrans) and the deformation can be fixed below this temperature. As a result, when they are heated above Ttrans their original shape can be recovered automatically. Most of the shape memory effects are based on the existence of separated phases related to the coiled polymer structure, cross-links, hydrogen bonding, etc. Chains tend to go back to the random coiled configuration, unless they are constrained by permanent or temporary cross-links and partial bonding. The shape memory transformation depends on the mechanism by which polymer molecules transpose between the constrained and random entangled conformations. By making use of this change, the shape memory effect can be appropriately engineered (Behl, M., Zotzmann, J., and Lendlein, A. 2012).
Shape memory polymers are processed by conventional methods to have a permanent ‘parent’ shape in the molding (or spinning) process. This permanent shape is stored in the system while the polymer assumes different temporary shapes, and by heating the polymer higher than the transition temperature, the permanent shape can be restored. The phase corresponding to the higher transition polymer component acts as the physical cross-links responsible for the permanent shape. The second component acts as a molecular switch and helps to freeze temporary shapes below the transition temperature, with either the glass transition temperature (Tg) or the melting temperature serving as the transition/switching temperature. Chemical cross-links can also be used in elastomeric shape memory polymers, instead of physical cross-links. Temporary shapes can thus be formed above the switching temperature and can be frozen by keeping the material below the switching temperature, while the permanent shape can be obtained again by heating above the switching temperature. This is the reason why most of the thermally induced SMPs have a one-way shape memory effect: they remember one permanent shape formed at the higher temperature, while many temporary shapes are possible at lower temperatures for which the systems do not have any memory. In terms of chemical structure, SMPs can be considered as phase-segregated linear block copolymers having a hard segment and a soft segment. The hard segment acts as the frozen phase and the soft segment acts as the reversible phase. The reversible phase transformation of the soft segment is responsible for the shape memory effect (Weiss, R. A., Izzo, E., and Mandelbaum, S. 2008).
Dynamic mechanical analysis (DMA) is becoming more and more commonly seen in the analytical laboratory as a tool rather than a research curiosity. However, DMA supplies information about major transitions as well as secondary and tertiary transitions not readily identifiable by other methods. It also allows characterization of bulk properties directly affecting material performance.
DMA can be simply described as applying an oscillating force to a sample and analyzing the material's response to that force (FIG. 2 (Prior Art)). From this, one calculates properties like the tendency to flow (called viscosity) from the phase lag and the stiffness (modulus) from the sample recovery. These properties are often described as the ability to lose energy as heat (damping) and the ability to recover from deformation (elasticity) (K. P. Menard, 1999).
The DMA supplies an oscillatory force, causing a sinusoidal stress to be applied to the sample, which generates a sinusoidal strain. By measuring both the amplitude of the deformation at the peak of the sine wave and the lag between the stress and strain sine waves, quantities like the modulus, the viscosity, and the damping can be calculated
It was suggested that the chemical crosslinks which remember the distribution of the physical crosslinks play a critical role in the shape restoring process (Hirai, T. et al., 1992). Hydrogels prepared by repetitive freezing and thawing of PVA aqueous solution were chemically cross linked with glutaraldehyde. The chemically cross linked hydrogel hardly changed its physical appearance, and showed good elasticity and strength as original gel. The melted gel showed shape memorizing property, that is, it could firmly hold nearly 200% of strain, keeping its original high elasticity. The strain could be released very quickly (<1 second) in boiling water, and the gel was suggested to be applied to a new type of gel actuator. The gel has attracted much attention as a material of artificial muscle or a gel actuator because of its quick reaction against environmental change. The gel thus obtained has excellent shape memorizing properties. The major differences from the so-called shape memorizing plastics are high water (or solvent) content and high elasticity, which are also the characteristics of the original gel.
Haiyan Du and Junhua Zhang (Du, H. and Zhang, J. 2010) reported the preparation of shape memory polymers based on poly(vinyl alcohol) (SM-PVA) cross-linked with glutaraldehyde. The authors discussed the influence of water content on the prepared materials. As PVA is a hydrophilic polymer, it has a small number of water molecules exposed to air, and the water molecules are helpful for shape memory characteristics. Shape memory behavior of SM-PVA, depending on the switching of chain segments, occurred at around Tg. Thermo-mechanical cycle test was performed to investigate shape memory properties, including the percentage shape recovery, shape recovery ratio, and percentage shape fixity. The authors asked two interested questions. One is whether shape recovery will take place by immersing SM-PVA in water, one good solvent of PVA. The other is whether other organic solvents of PVA will stimulate the shape recovery. To answer the two questions, they selected a series of solvents. Good solvents of PVA (water and dimethyl sulfoxide, DMSO) and several organic, poor solvents of PVA (methanol, acetic acid, tetrahydrofuran (THF) etc.) were tested. The thermal analysis results show that there is small number of water molecules in SM-PVA, and the water molecules actively affect shape memory behavior. The sharp drop of the storage modulus and the loss tangent delta confirmed that the studied material have characteristics of shape memory polymer, with the switching temperature at around Tg.
Hassan and Peppas (Hassan, C. M. & Peppas, N. A. 2000) described the preparation of shape memory material based on chemically cross-linked PVA to make good use of the excellent properties of PVA and to widely explore applications of PVA. The cross-link network and undisturbed crystal domain in PVA are considered as the fixed phase, while the amorphous phase acts as the reversible phase. Due to the advantages of nontoxic nature, bio-compatible and biodegradable applications, good mechanical property, etc., SM-PVA has potential use in many fields. Prepared gels were characterized in terms of their swelling and dissolution behavior, degrees of crystalinity, and crystal size distributions. In addition, the long-term stability was addressed in order to consider the appropriateness of such materials for long-term biomedical or pharmaceutical applications. The overall structure and stability were examined in terms of water content, fractional PVA dissolution, degree of crystalinity, and crystal size distribution. Results indicate that an increase in PVA chain length and an increase in the free volume within the network together allow for secondary crystallization to proceed as the material swells. Secondary crystallization was more pronounced for more loosely cross-linked samples. An increase in the free volume and mobility within the network allowed for additional crystallization to proceed during swelling.
Preparation of hybrid membranes based on PVA and MWCNTs by casting method was described by (Samal, S., et al., 2009). The morphological analysis using SEM, thermal and de-swelling behavior gave information about PVA thermal stability and de-swelling and phase transition and state of water inside of hybrid membranes. The effect of MWCNTs in the hybrid membranes was more significant when its concentration was high. The thermo degradation (Td) and crystallization (Tc) temperatures of PVA increase by 10 and 9° C., respectively in presence of 50×10−2 w v−1% of MWCNTs. Besides, the amount of non-free water increases with increasing of MWCNTs probably due to a capillary effect. SEM micrographs showed the presence of MWCNTs agglomerates characterized by a 200-300 nm size. These agglomerates would be one of the factors influencing the amount of non-bound and bound water into the hybrid membranes. The moisture of PVA hybrid membranes in equilibrium with that of ambient increased with increase of MWCNTs content going from 7 to 16 wt. %. The behavior of swelling and de-swelling data suggested that the ionic surfactant used to disperse MWCNTs in water has an important role in the hybrid membranes as explained by some apparent contradictory results.
However, none of the above-discussed references discloses or suggests a robust SMP composition. Accordingly, there exists a need in the art to overcome the deficiencies and limitations described herein above.